The present invention relates to a radio propagation estimating program, a radio propagation estimating method, and an apparatus executing this method that are used in the layout design of a base station in a wireless LAN system, a mobile phone/PHS system, and a broadcasting system for analyzing a radio propagation path in a predetermined space, and more particularly to a radio propagation estimating program, a radio propagation estimating method, and an apparatus executing this method for analyzing the power of a radio wave that arrives at a receiving point.
Recently, a wireless network system such as a mobile phone, a PHS, a wireless LAN, and a DSRC (Dedicated Short Range Communication) finds many applications as more and more infrastructure facilities are built and the costs of apparatuses are lowered. In such a wireless network system, the radio propagation characteristics greatly affect the data communication performance due to factors such as the layout, structure, material, and reflection of the buildings and, therefore, it is necessary to estimate in advance how the radio wave propagates in the space and how far the radio wave can travel.
In general, a radio wave originated from a transmitting antenna in a point in a space travels directly in all directions with the transmitting point as the center. If there is an obstruction in the direction into which the radio wave travels, the radio wave reflects on, passes through, or diffracts around the obstruction, scatters in many directions, and is attenuated. The physical phenomenon of those electromagnetic waves is generated all according to the Maxwell's equation. To solve the Maxwell's equation on a computer, the approximation method must be changed according to its problem size.
For example, a simulation of a broadcasting wave coverage range requires a simulation of a range from several kilometers to several hundred kilometers. In addition, because there are weather conditions and many other factors potentially affecting the radio propagation in this case, the conventional method is that a mathematical expression, statistically obtained from the actual measurement, is used to estimate the power required to cover an intended range according to a distance between the transmitting point and the receiving point.
On the other hand, to simulate a radio propagation path in a DSRC or a wireless LAN where the radio wave coverage distance is limited to a relatively short distance, not the probabilistic method described above but a deterministic method is used to find the radio propagation path. One of the deterministic methods is a ray tracing method (light ray tracing method) in which the propagation of electromagnetic waves is calculated as a propagation similar to that of light. The known methods of this ray tracing method are an imaging method (mirror image method) and a ray launching method.
The calculation principle of the imaging method will be described with reference to FIG. 13.
Assume that there are two object surfaces, a ceiling surface 3 and a floor surface 4, on which a radio wave is reflected in the target space. In this case, there are the following paths via which the radio wave propagates from a transmitting point 1 to a receiving point 2 within two reflections: (1) a path via which the radio wave arrives directly from the transmitting point 1 at the receiving point 2 (2) a path via which the radio wave is reflected once on the ceiling surface 3 or on the floor surface 4 before arriving at the receiving point 2 (3) a path via which the radio wave is reflected on the ceiling surface 3 and then on the floor surface 4 before arriving at the receiving point 2, and (4) a path via which the radio wave is reflected on the floor surface 4 and then on the ceiling surface 3 before arriving at the receiving point 2.
The following describes how to calculate a path (4), that is, the path via which the radio wave is reflected on the floor surface 4 and then on the ceiling surface 3 before arriving at the receiving point 2.
First, with the floor surface 4 as a mirror surface, the positions where the receiving point 2 and the ceiling surface 3 are reflected as mirror images are calculated. Let those positions be a mirror image 5 of the receiving point with respect to the floor surface and a mirror image 7 of the ceiling surface with respect to the floor surface, respectively. In addition, with the mirror image 7 of the ceiling surface as a mirror image surface, the position where the mirror image 5 of the receiving point with respect to the floor surface is calculated and this position is set as a mirror image 6 of the receiving point with respect to the floor surface/ceiling surface. After that, a straight line is drawn from the transmitting point 1 to the mirror image 6 of the receiving point with respect to the floor surface/ceiling surface to find the intersection between the straight line and the floor surface 304 and between the straight line and the mirror image 7 of the ceiling surface with respect to the floor surface. The propagation path in the real-image world can be found by returning those intersections back to the real-image world by reversing the procedure for finding the mirror image of the receiving point.
To actually find a propagation path using the imaging method described above, the following procedure is used.
First, the permutations of n object surfaces of the reflection candidates from all object surfaces (m) in the target space are calculated and the result is set as the reflection path candidates. The number of permutations, in other words, the number of reflection candidates, is mPn. The mirror image calculation described above is performed for the reflection path candidates to find image reflection points and, based on the image reflection points, the reverse mirror image calculation is performed to find the reflection points in the real space. Joining the reflection points with a line produces a propagation path calculated in the imaging method. In the imaging method, the processing described above is repeated the number of times, from one reflection to a predetermined maximum number of reflections N, to find all propagation paths.
Next, the calculation principle of the ray launching method will be described with reference to FIG. 14.
In the ray launching method, rays are generated from a transmitting point 1 in many directions. When generating rays in this method, rays may be generated either evenly in many directions or according to the directional characteristics of the transmitting antenna. The ray launching method searches for an object that exists in the traveling direction of each ray and, if any, finds the intersection with the object, changes the direction of the ray to the mirror surface reflection direction with the intersection as the reflection point, and repeats the search for an object in the ray traveling direction. Rays 11 and 12 shown in FIG. 14 have no object in their traveling directions. On the other hand, ray 13 is reflected on the floor surface 4, changes its direction, is reflected again on a ceiling surface 3 existing in its traveling direction, and changes its direction again. Because there is a receiving point 2 in this case, it is found that the ray 13 is one of the propagation paths. Ray 14 is found to be a direct wave because it arrives at the receiving point 2 that is in its initial generation direction. Because a ray 15 is reflected on the floor surface 4 but there is no object in its traveling direction that has been changed, the ray search is terminated at this point. The ray launch method finds propagation paths as described above.
Although the calculation load of the ray launching method, in which the calculation load increases linearly as the number of object surfaces increases, is lighter than that of the imaging method in which the calculation load increases exponentially as the number of object surfaces increases, the ray launching method has a disadvantage that the radio propagation paths cannot be found accurately. For example, even if rays are generated at the transmitting point and arrive at the receiving point, the paths of some rays cannot be found. To solve this problem, a large receiving area 8 (FIG. 14) including the receiving point is set up to find rays arriving at this receiving area, with the result that the ray paths not actually arriving at the reception point are found. The reduction in accuracy is proportional to the distance between the transmitting point and the receiving point.
On the other hand, the imaging method that, in principle, finds the propagation paths from the transmitting point to the receiving point accurately is an extremely accurate calculation method. However, the calculation time increases exponentially as the number of object surfaces increases as described above, because the calculation time is proportional to a number represented by the permutations of the number of object surfaces of reflection candidates from the number of all object surfaces.
To solve this problem, a technology is disclosed in JP-A-10-62468 shown below for reducing the calculation load while maintaining the calculation accuracy of the imaging method.
This technology narrows down the object surfaces in the target space only to those viewable from both the transmitting point and the receiving point to reduce the calculation time of the imaging method.
It should be noted that, if the distance between the transmitting antenna and the receiving antenna is too large when the ray tracing method is used, ray approximation cannot sometimes be performed due to random events probabilistically caused by weather conditions or moving objects or due to diffraction generated by the roundness of the Earth.
In such a case, an approximation expression is statistically prepared from the measurement value of the power arriving at a receiving point, and the receiving power is calculated by changing the parameters of the approximation expression or the approximation expression itself according to the conditions (mountains, cities, etc.) existing between the transmitting antenna and the receiving antenna. However, it is difficult to accurately calculate the power, which arrives the receiving point, by this method.
JP-A-2001-28570 described below discloses a method for finding an arriving power as accurately as possible in which an experimental formula based on the rule of thumb is applied to the propagation paths obtained by the ray tracing method and, based on the result, the electric field intensity at the receiving point is estimated.